(1) Field of the Invention
This invention relates to a coal-methanol slurry having excellent storage stability, transportability and combustibility and a process for the production of the slurry.
(2) Description of the Prior Art
People are more and more concerned these days with the exhaustibility of petroleum resources which have been used widely as energy sources. Coal has thus started attracting people's attention back as a substitute for petroleum. As a solid fuel, coal is however accompanied by inherent and unavoidable disadvantages in transportation and handling compared with liquid fues.
As a method for solving this problem, it is attempted to pulverize coal and then mix it with a medium such as mineral oil or water, in other words, to convert coal into a slurry fuel. Mineral oil, water, methanol and the like have been primarily investigated as media for use in the practice of this method. These different media have brought about different characteristic features.
Among resulting slurry fuels, coal-methanol slurry (hereinafter abbreviated as "CMS" for the sake of brevity) has such advantages as will be described below and methanol has hence drawn interests as a medium for the fluidization of coal.
Methanol can now be produced from a wide variety of different raw materials. CMS may be economically produced from coal only, provided that an economical synthesis route making use of coal gasification can be developed for methanol in the future. Use of methanol as a medium makes it possible to cut down the transportation cost per unit energy compared with aqueous slurry. Since the freezing point of methanol is low (-98.degree. C.), there is no danger for CMS to freeze even in cold districts. This CMS method can be applied even to low-grade coal which contains lots of water, involves the danger of spontaneous combustion depending on its type and is not used widely at present. At a certain suitable location along the transportation route of CMS or at a place of consumption of CMS, a portion of methanol may be separated and then reused as a transportation medium or as methanol for various applications such as a fuel and a raw material for the chemical industry.
As a prior art technique for CMS, the process disclosed in Japanese Patent Laid-Ooen No. 55304/1978 has been known. In this prior art process, coal is ground until most of its particles are reduced to 100 mesh or smaller. The resultant particles are mixed with methanol to obtain a methanol-pulverized coal suspension. This suspension is pseudo-thixotropic, and can maintain its suspended state provided that it is gently stirred during its storage. It can retain its suspended state without allowing its components to separate out even when it is pumped through a pipeline, namely, has shear thinning rheology. It can therefore be pumped for its transportation at an apparent viscosity lower than its viscosity at standstill. When it is stored without stirring, a part of the suspended pulverized coal precipitates on the bottom of its storage vessel and forms a hard coal layer, namely, a so-called compact layer. Once this compact layer is formed, it is difficult to bring the suspension back into the original uniform suspension. The above suspension is thus not considered to have sufficient transportability and storage stability.
Japanese Patent Laid-Open No. 45283/1983 discloses a process featuring an addition of water in a suitable amount to obtain CMS which is less susceptible to compact layer formation and has good storage stability. This process is however still insufficient as to improvements to the transportability.
In general, the quality of coal slurry is evaluated by characteristic properties such as stability (storability), viscosity (transportability), combustibility, etc.
As parameters governing these characteristicproperties in CMS, may be mentioned the type of the coal, the concentration of the coal in the slurry, the particle size distribution and water content of the coal, and additives. The quality of CMS is determined by these parameters along with certain production conditions such as ginding and mixing. These parameters however give influence to one another and act in a complex manner. It is therefore not easy to find out a combination of suitable conditions for the respective parameters. In conventional processes, no sufficient investigation has been made on this point. In order to obtain CMS of good quality stably from various types of coal, it has hence been required to find out optimum conditions for the respective parameters whenever the coal is changed from one type to another.
Regarding the particle size distribution of coal in coal slurry, a great deal of work has been made on coal-water slurry (hereinafter abbreviated as "CWS" for the sake of brevity), including for example Japanese Patent Disclosure No. 501183/1983. However, there is no report on the same subject matter investigated on CMS. In the case of CWS, the compatibility between coal and water is generally low and no interaction is usually allowed to take place therebetween. Hence, its analysis is relatively easy. In the case of CMS, -large miscibility exists between coal and methanol. Some components of the coal are partly eluted in the methanol, developing such complex interaction that the compositions of both solid and liquid components in CMS are modified and the methanol penetrates into pores and cracks in the coal to facilitate the disintegration of the coal. As has been mentioned above, CMS shows different behavior from CWS. Turning next to their combustion, different from coal particles in CWS in which water is hard to penetrate in coal and has no combustibility, coal particles in CMS are allowed to burn while being disintegrated into particles owing to their deflagration by expansion and combustion of the methanol penetrated therein.
Since CMS shows substantially different behavior from CWS as mentioned above, it is necessary to analyze parameters governing the storage stability, transport-parameters ability, combustibility and the like of CMS from a viewpoint different from that applied to CWS so that optimum conditions be established for its production.